If your output does not display an RPM for your CPU fan, and you are positive it is running, you need to increase the fan divisor. If your fan speed is shown and higher than 0, skip the next step.
Increasing fan_div:

The first line of the sensors output is the chipset your motherboard uses to read the speeds/temps/voltages. Make a backup first:
Code:

cp /etc/sensors.conf /etc/sensors.conf_original

Edit the /etc/sensors.conf file as root

nano /etc/sensors.conf

and look up your exact chipset. The names all look alike, so make sure the one you are editing is yours. Add the line fanX_div 4 near the start of your chipset config. Replace the X with the number of your CPU fan's, for me that was 2. You have to figure out for yourself which one it is, but it's probably 1, 2 or 3.

Save, and run:

sudo sensors -s

which will reload the sensors.conf's set variables.
Run sensors again and check if there is an RPM readout. If not, increase the divisor to 8, 16 or 32. YMMV!

You can safely ignore anything that's not fanX_div. I would advise you to leave the other default settings as they are.

pwmconfig

(If you are a very cool hacker, you might want to skip this section and write /etc/fancontrol by your own, which also saves you from hearing all your fans at full speed)

Once you have lm sensors properly configured, run pwmconfig to test and configure speed control of your fans:

pwmconfig

Follow the instructions in pwmconfig to set up basic speeds.

The default configuration options should create a new file, /etc/fancontrol.

Follow the instructions in pwmconfig to set up speeds.

Tweaking

Second warning: Some of the steps outlined below describe how to tweak fan speeds. Before doing this be sure you have a low cpu load and are comfortable playing around. If at any time during tweaking you notice the CPU temperature start to rise dramatically, do a echo "255" > /sys/class/hwmon/hwmon0/device/pwm1 to spin up the fan all the way until things cool down. Basically, you should know what you're doing before fooling with the configuration file.

Note: On several systems, the included script may report errors as it trys to calibrate your fan to the PWM. You may safely ignore these as errors. The problem is that the script doesn't wait long enough before ramping up or down the PWM.

If you want more control, you will probably need to tweak the generated configuration. Here is a sample configuration file:

INTERVAL: how often the daemon should poll cpu temps and adjust fan speeds. Interval is in seconds.

The rest of the configuration file is split into (at least) two values per configuration option. Each configuration option first points to a PWM device which is written to which sets the fan speed. The second "field" is the actual value to set. This allows you to monitor and control multiple fans and temperatures (if your pc supports it).

FCTEMPS: The temperature input device to read for cpu temperature. The above example corresponds to /sys/class/hwmon/hwmon0/device/temp1_input.

FCFANS: The current fan speed, which can be read (like the temperature) in /sys/class/hwmon/hwmon0/device/fan1_input

MINTEMP: The temperature (°C) at which to SHUT OFF the cpu fan. Efficicent CPU's often will not need a fan while idling. Be sure to set this to a temperature that you know is safe. Setting this to 0 is not reccommended, use a sane value.

MAXTEMP: The temperature (°C) at which to spin the fan at it's MAXIMUM speed. This should be probably be set to perhaps 10 or 20 degrees (°C) below your CPU's critical/shutdown temperature. Setting it closer to MINTEMP will result in higher fan speeds overall.

MINSTOP: The PWM value at which your fan stops spinning. Each fan is a little different. Power tweakers can cat different values (between 0 and 255) to /sys/class/hwmon/hwmon0/device/pwm1 and then watch the cpu fan. When it stops, use this value.

MINSTART: The PWM value at which your fan starts to spin again. This is often a higher value than MINSTOP as more voltage is required to overcome inertia.

There are also two settings fancontrol needs to verify the configuration file is still up to date. The lines start with the setting name and a equality sign, followed by groups of hwmon-class-device=setting, seperated by spaces. You need to specify each setting for each hwmon class device you use anywhere in the config, or fancontrol wont work.

DEVPATH: Sets the physical device. You can determine this by executing the command

For Dell Latitude/Inspiron laptops, you may want to use i8kutils/i8kmon.

simple bash script to fine tune fan speed

Run this as root if you'd like to see how various pwm values translate into fan RPM. As you can see, this script assumes that you have fancontrol running and disables it for you, then re-enables it when you're finished. Enjoy.